Roofing and the like



Patented Apr. 29,1941

UNITED STATES PATENT OFFICE ROOFING AND THE LIKE Richard A. Wilkins,Rome, N. Y.,

assignor to Revere Copper and Brass Incorporated, Rome, N. Y., acorporation of Maryland No Drawing. Application October 30, 1940, SerialNo. 363,515

3 Claims.

or cold rolling the slabs into sheets of the de-- sired thickness, andfinally rolled sheets to a dead soft temper.

The copper for roofing has been refined in various ways,electrolytically refined copper, which commonly is 99.9% pure copper,being used almost exclusively during the past forty years. In refiningcopper for these purposes it has heretofore always been the practice ofproducers thereof to remove so far as possible all impurities so as tosecure as pure a copper as possible. The known high resistance of purecopper generally to chemical corrosion, and the observed fact thatimpurities commonly found in rolled copper tend to make it less ductilein both the hot and cold rolled conditions, more difiicult to produce byeither hot or cold rolling, and commonly "cold short in the sense thatit tends to crack when bent or otherwise cold worked, have caused suchpure copper heretofore to be considered the most satisfactorycommercially available material for light weight metal roofing and thelike.

However, it is, a frequently observed fact that copper roofing, evenwhen substantially 100% pure electrolytically refined copper, oftenfails soft annealing the in use, being subject to cracking sufficient torender it non-waterproof. Heretofore it was believed that these crackswere caused bymechanical fatigue resulting by reason of the cyclicstresses set up by alternate expansion and contraction of the coppersheets under varying temperature conditions. For, this. reason buildershave resorted to expensive roof constructions, and

various expensive expedients 'such as corrugating the roofing and theemployment of elaborately designed expansion joints, and the use ofheavy gauge copper in the attempt to relieve or reduce the eiiect on theroofing-of these cyclic stresses and thereby eliminate so far aspossible this form of failure. Although these attempts have beenpartially successful, the expense incidental thereto, and the manyfailures which actually have occurred in spite of these precautions,have seriously interfered in recent years with the general adoption ofthis otherwise very satisfactory Prolonged investigations by applicantcovering a period of years have demonstrated that the reason for thefailure of copper roofing and the like heretofore has not been correctlyunderstood.

This material, he has found, does not fail primarily by reason of thecyclic stresses to which it is subjected when in use, as heretoforebelieved,v

but, strangely enough, primarily by reason of a heretofore whollyunsuspected type of corrosion which occurs when this highly pure copperis exposed to an atmosphere contaminated by wind borne products ofcombustion of modern domestic and industrial fuel burning appliances andindustrial atmospheres in general.

Applicant has found, that upon removing the patina and other accumulatedcorrosion products from copper roofing which has failed, the coppersurface thus exposed is characteristically slightly pitted, instead ofbeing in its smooth unpitted condition in which it originally left therolling mill or the metal showing a uniform decrease in thickness overits whole surface due to corrosion.

His investigations indicate that these slight pits are caused by theformation onthe copper, when exposed to a contaminated atmosphere, of afilm of corrosion products which is characterized by a lack of adhesionto the copper, a lack of tenacity in respect to failure, and a tendencytoward perviousness which allows the copper to be attacked locallyrather than uniformly by the corrosive media and thereby result in theformation of sharply defined inwardly pointed pits inthe nature ofnotches. Copper so attacked rapidly i'ails when subjected to the cyclicstresses hereinbefore referred to by reason of the natural. tendency ofthe stresses to be concentrated in the metal below the bottoms of thenotch-like pits. As a result-of this concentration of the stresses thefatigue strength of the material is exceeded at the pits and the metalcracks.

Pure copper roofing as heretofore compounded applicant has found iscomparatively sensitive to this "notch effect when caused by acombination of chemical pitting and subjection to cyclic stresses. Thissensitivity, it has been found, is very closely correlated tosensitivity to work hardening of the material when cold worked, eachvarying directly with the other. When pure copper roofing is cold workedit has been found working will occur at the pits when pitted copperroofing is subjected to the above mentioned cyclic stresses, whichstresses may bend the material alternately back and forth, alternatelystretch and compress it, or otherwise work it. On account of this, andthe character of the notchlike pits, the resistance to cracking or otherfatigue strength of the pure copper roofing, when subjected to cyclicstresses and to a contaminated atmosphere causing the pits, is reduced,it has been found, to as much as 20% of that of the original material asit left the mill. 3

Applicant has-found that the above type of failure of copper roofingasconstructed from thin sheets, commonly from about'0.003 to 0.03

inch in thickness, can be substantially entirely avoided byincorporating into the copper controlled amounts of arsenic, whileotherwise maintaining the substantial purity of the copper by takingsuch precautions as will avoid the presence of certain deleteriousconstituents which might be entered into the copper with the arsenic orotherwise.

With the improved copper roofing it has been of chemical corrosion andsubjection to cyclic stresses.

Further, the improved roofing as compared to prior copper roofing may,by proper control of the arsenic content, be made markedly lesssensitive to notch effects occasioned bythe presence of scratches, dentsor other abrasions usually in.- troduced during handling of the roofingor its installation. This results as a consequence of the presence ofthe arsenic, when in proper amount, rendering the roofing more immune tocracking when subjected to cyclic stresses.

It has been demonstrated by laboratory experiments and fleld tests thatthe presence of the proper amount of arsenic in the copper roofingproduces on its surface a corrosion film which is tenacious, adherent,impervious and non-hygroscopic, the film being comparably as ductile asthe copper base material itself, thus permitting the roofing to berepeatedly bent without rupture of the film or interruption of itsadherence to the copper. Further because of these properties the filmeffectively protects the underlying base material fromcontaminated airand moisture, and thus effectively eliminates the possibility of theformation of the above mentioned pits.

This highly protective film, which forms in the matter of a few minutesor hours after exposure of the improved roofing, applicant believes iscaused by an electro-chemical reaction. The film chemically is largelycomposed of arsenic compounds which are inherently more noble thaneither pure copper or compounds formed by the corrosion of pure copperwhen subjected to a contaminated atmosphere. Because of thischaracteristic of the film and its continuity and imperviousness itprevents the continued corrosion which occurs in pure copper roofing;after initial corrosion by a contaminated atmosphere forms thereon afilmof corrosion products, be-

cause with pure copper the film is loose, friable and incompletelyadherent; and because of this is in effect less noble than purecopper.Furthermore this protective film formed on the improved roofing has theproperty of being essentially self-healing in that if for any reason itscontinuity is locally interrupted it will substantially immediatelyreform and thus act to retard local corrosion at the interrupted regionswhich in a pure copper would continue to corrode at an accelerated rateresulting in formation of pits and as a consequence serious loss offatigue strength of the roofing.

It will therefore be understood that arsenic is added to the roofingprimarily for the purpose of increasing its corrosion resistance, forpreventing it from pitting when subjected to atmospheric corrosion, andfor decreasing its sensitivity to failure by notch efiect" under thecyclic stresses to which roofing is subjected.

Appreciable effects in the above respects will be observed with aslittle as about 0.02% arsenic. The maximum efiect in respect to theprimary purposes for which the arsenic'is added is had with about 0.5%arsenic, with which amount the protective film will form in a practicalsense substantially immediately on and be distributed continuously anduniformly over the exposed metal and thereby act effectively to preventfurther corrosion of any kind. As the arsenic is increased above about0.5% some effect in respect to increased resistance to corrosion andpitting is, observed, an increase in the arsenic to 1% resulting in anincrease of only about 10% in these resistances. However, as the arsenicis increased above 0.5% the high insensitivity to notch effect rapidlydecreases, the roofing tending to become more sensitive to "workhardening and as a result more sensitive to failure when subjected tocyclic stresses. Therefore an amount ofarsenic in the neighborhood of0.5% is preferably employed, say an amount within the range of 0.35 to0.65%. For some purposes, say where the tendency to pit would be greaton account of severe contamination of the atmosphere, larger amounts ofarsenic may be employed, say up to l or 1.5%, which would cause theprotective film to contain a, larger percentage of arsenic compoundsthan in the case of a lower arsenic content of the roofing with theresult that the film would be more resistant to the corrosive media.Such improvement however in the film and resistance of the roofing tocorrosion would be gained. at a slight sacrifice of the resistance tonotch efiect, although in this latter respect the roofing with thehigher amounts of arsenic would still be superior to pure copperroofing.

Applicant has found that best results ordinarily will be secured whenthe roofing does not contain impurities which tend to render it lessmalleable in the hot and cold conditions and thus tend to cause the hotor cold rolling or other working of the roofing during its fabricationto result in sub-microscopic surface cracks therein analogous in theireflects to pits.

Among these deleterious substances are lead,

bismuth, iron, nickel and sulphur, all of which tend to make the copperroofing either more difficult to produce or less resistant to thecombined effects of cyclic stresses and corrosion. More than about0.002% lead or 0.001% sulphur tends to make the material deleteriouslyless malleable in thehot condition. More than a trace of bismuth tendsto make it deleteriously less malleable in both the hot and coldconditions. More than about 0.005% iron or nickel tends to make it moresensitive to work hardening and in consequence more sensitive to notcheffects. Further, more than above these amounts of bismuth, sulphur,iron and nickel acting individually or in combination with each othercauses arsenic-bearing copper roofing to approach the poor resistance ofpure copper roofing to corrosion and its sensitivity to notch effects,

I and thereby act to destroy the beneficial efiects will be understood,that in the sense the copper roofing does not contain deleterioussubstances or amounts thereof which substantially destroy the beneficialefiects tended to be imparted by the arsenic, the balance of the alloyin respect to the arsenic is essentially copper.

In making the copper alloy roofing according to the invention the coppermay be melted, and the arsenic added to the melt in'the form of acopper-arsenic alloy, rich in arsenic. The molten metal may then be castinto slabs, which latter may be rolled into sheets of desired thicknessby the usual mill process of rolling copper sheets. Finally therolled-sheets are annealed to the degree of softness that will bestresist the cyclic stresses to which they are subjected when in use. 7

For convenience in terminology the article of manufacture according tothe invention is termed roofing, it being understood that as used in theappended claims such term includes various shingles, sheathings,valleys, and like protective an extensive external roofing surface, thecopper being alloyed with such amount of arsenic between 0.02 and 1.5%as will cause the formation on said surface when exposed to atmosphericcorrosion of a self-healing, adherent, protective film composedpredominantly of arsenic compounds, while rendering the sheet copperless sensitive to failure by reason of notch efiect than sheet copperwithout the arsenic when subjected to the cyclic stresses caused byexpansion and contraction of the roofing.

2. Roofing comprising sheet copper presenting an extensive externalroofing surface, the copper being alloyed with approximately 0.02 to1.5% arsenic, the balance of the alloy in respect to the arsenic beingessentially copper.

3. Roofing comprising sheet copper presenting an extensive externalroofing surface, the copper being alloyed with approximately 0.35 to0.65% arsenic, the balance of the alloy in respect to the arsenic beingessentially copper.

RICHARD A. WILKINS.

